Naish Nalu 10'10" vs Nalu 10'6.5"

Here is the story, I bought a Naish Mana 9.6 for my girlfriend and we both love it.
After a few paddles on my own I feel the Mana lacks a bit of tracking and I start to think about a board for myself (78kg, 180cm) something longer more cruise and workout orientated, something I can use with the GF when I can't pull out my windsurf gear.
We live about 300 meters from the water and the whole summer the conditions are flat water. In the winter good surf conditions though.

I compare the volume of the Mana (190L) and find a board that is very close, the Nalu 10"10 (201L). Looking at the multi concaves I think OK looks perfect for flatwater and tracking so I get the board. Go out first time, big difference with the Mana. Next time go out with both boards to compare and the Nalu feels like a tank. Big like a train on rails. And man, it's heavy!! I start to wonder if I made the right choice.. It just feels too stable. The Mana is more fun and lively turns. Ok I get more strokes on the sides and the Nalu glides and tracks like a rocket but I am afraid that I will outgrow the board in no time.

We got out in very small waves yesterday with both boards and while we both had much fun the Nalu did good… but for how long?
I got the board about a week now. I could sell it and get something a bit more lively that still tracks good in flatwater.
I read everybody likes the Nalu 10"6.5

I understand now that the Nalu 10"10 is more to take a kid or a dog in tandem or go fishing with a frigobox on the bow?
Not really what I had in mind..
Will I be more happy with the 10"6.5?
Does it really surfs that much better??
I am afraid now the multi concave is not the best idea to get some fun in surf.

Anybody has experience with both boards?

Will I be more happy with the 10"6.5?
YES!
Does it really surfs that much better??
YES!

Just on that, do you keep it as a thruster on flat water or do you convert to single fin

Thanks slab, appreciated